This invention relates to a brake control valve for vehicle brakes.
U.S. Pat. No. 3,163,007 shows a brake valve with a piston which moves in a stepped cylindrical bore of a valve housing against the force of a spring. The piston includes port control lands which control communication between a brake pressure chamber connected to the vehicle brakes, a reservoir, and a pressure source, so that with the support of the pressure source, a brake pressure is developed in the brake pressure chamber which is a function of the actuating force applied to the piston. Furthermore, the brake control valve contains an annular piston which is sealed with respect to an enlarged section of the stepped cylindrical bore and which can be moved in the cylindrical bore by the piston after a pre-set stroke of the piston has been overcome.
This known brake valve also includes a control device which is sealed and is movable in a displacement chamber. At its forward end in the displacement chamber the control device encloses a pressure chamber. A pressure reservoir is formed by an annular groove formed in a rear end of the control device which is connected with a source of pressure. The pressure reservoir is bordered by a port control edge of the control device. When the control device is moved by a manually operated control rod against the force of a spring, then the port control edge opens a passage between the pressure reservoir and a connecting line which is connected to the pressure chamber and to the brake cylinders. Simultaneously, a connection between the pressure chamber and a reservoir is closed by a further port control edge. Furthermore, the brake control valve contains an annular piston which is movable in an enlarged section of the pressure chamber which is located at a distance ahead of the control device and that is sealed with respect to the displacement chamber. When the distance is overcome by movement of the control device, then a rubber seal fastened to the end face of the control device engages the annular piston so that it follows the further movement of the control device.
This known brake control valve is a mass-flow supported brake control valve that opens a flow of pressurized fluid from the pressure source to the brake cylinders and applies the brakes when a pre-set stroke of the control device is exceeded. Simultaneously, the pressurized fluid delivers a reaction force acting upon the control device which is proportional to the fluid pressure, which force opposes the actuating force applied to the control rod, which will be transmitted to the brake pedal and which thereby results in a very stiff brake system. This stiffness depends on the velocity of flow from the pressure source, but not on the elasticity of the power output, viscosity effects, etc. On one hand, the pressure chamber operates as a reaction force chamber, in which the reaction force is built up. On the other hand, if the pressure source fails, and as a result no reaction force is built up in the pressure chamber, then the pressure chamber operates as a displacement chamber, which delivers pressurized fluid to the brake cylinders when the control device is moved in a purely manual operation. Thereby emergency braking is possible even when the pressure source fails.
The operation of the known brake control valve is, however, not completely satisfactory during manual braking (emergency operation upon failure of the pressure source), since the same cross section is employed both for the filling of the brakes as well as for the build-up of the brake pressure itself. It is desirable that a larger cross section be effective during the filling than during the build-up of the brake pressure, since during the filling a comparatively large amount of fluid with low pressure must be conveyed while a small amount of fluid at high pressure is required during braking. Therefore, it is desirable that the brake control valve be configured as a two-stage unit for emergency braking operation. Such a two-stage brake control valve is described in U.S. Pat. No. 4,963,692, issued Oct. 16, 1990 and assigned to the assignee of this application.